1. Field Of The Invention
The present invention relates generally to a circuit board which is coupled to a local area network peripheral (e.g. a printer) and which allows the peripheral to be an intelligent, interactive network member eliminating the necessity of dedicating a personal computer to manage the peripheral. More particularly, the present invention relates to a method and apparatus for providing an interlock mechanism for preventing inadvertent application of a load enable signal to a PROM, such as when power is applied or disconnected from the board.
2. Related Art
Local Area Networks ("LANs") are known for coupling together a plurality of personal computers with peripheral devices such as printers, copiers, etc., to provide for enhanced communication and shared resources. Heretofore, peripherals such as printers coupled to a LAN were rather unintelligent, merely accepting information from the LAN and printing such information on a hard copy. Moreover, such printers usually required a host personal computer ("PC") to effectively manage the flow of data to the printer, i.e., to act as a "server" for the printer. This almost always required that the host PC be dedicated solely to the printer server task.
A number of products have recently appeared which ostensibly eliminate the need for such a dedicated PC by incorporating hardware and software into a circuit board which may be coupled into the peripheral in order to perform limited server functions. For example, ASP Computer Products, Inc. provides a device known as "JetLAN/P" which acts as a stand-alone print server for Novell networks. The JetLAN/P.RTM. device couples to a LAN using a 10Base-2 thin coaxial cable or a 10Base-T twisted-pair cable. However, the JetLAN/P.RTM. couples to the printer only through the printer's parallel port. Thus, while print information can be sent to the printer, the amount of printer status information which can be returned from the printer is severely restricted. For example, such a device may obtain "off-line" and "out of paper" status from the printer, but little else. Such a device does very little toward making the printer a truly intelligent, responsive member of the network.
Other known devices for coupling a printer to a LAN include the Hewlett-Packard Jet Direct.RTM. C2071A/B and C2059A, the Extended Systems EtherFlex.RTM., the Intel NetPort.RTM. and NetPort II.RTM., the Castelle LANPress.RTM. and JetPress.RTM., and the MILAN FastPort.RTM.. However, all of these devices suffer from the same disadvantages as the ASP JetLAN in that they do not allow the printer to transmit sufficient amounts of data to the LAN to enable the printer to be an effective and intelligent member of the network.
Interactive peripheral devices, as discussed above, may include a processor, a random access memory (RAM) and a programmable read only memory (PROM). Due to recent developments in ROM technology, it is possible to provide interactive peripheral devices with an EPROM which can be electrically erased and electrically reprogrammed. The ability to electrically erase and electrically reprogram is a distinct advantage over the previous technology since PROMs had to be removed from a board, erased with ultra-violet rays and then reprogrammed from a separate programming station.
This advantage of electrically erasing and electrically reprogramming a PROM while on a board has not been entirely welcome. That is, an EPROM can be easily altered by corrupt data sent from a processor during power-up or power-down. That is, since most processors utilized today have a CMOS technology design, these processors continue to process as power dissipates upon power-down or starts to process during power-up with very little power. Because of this, transient data bursts may be sent to the EPROM during either an abrupt power-on or power-down. These transient data bursts can easily corrupt a program stored in an EPROM.
Heretofore, it has not been possible to prevent an EPROM from being loaded with corrupt data. Consequently, when power is removed or applied to a board, a program within an EPROM can be easily altered.